A problem I run into all the time, especially in the last five years or so, is the curious case of people forming opinions about things they have no experience with. The fact that people have opinions at all isn't just good, it's great! Thinking critically about things we come into contact with so we can conclude our own thoughts and discuss with others is incredibly valuable to society.
However, what is not helpful to society is when people form opinions of things they have no context for. This is a problem that impacts everything from how we view history to how we feel about new music, and even how we react to social turmoil. Allow me to provide you with a little 'for instance.' Say that a brand new music album has come out from a popular singer. Listening to that album and deciding you don't like it would be an appropriate way to form an opinion of the material. You could even decline to listen to based on previous impressions of that artist's work, saying that you know from experience you don't care for them. Those are perfectly valid opinions to offer, and can lead to productive discussion. Offering those opinions insultingly and belligerently is not at all beneficial, but that's a problem to address another time.
If, on the other hand, you don't listen to the album and/or have no knowledge of the artist, informing everyone that you think the music is awful lends you no credibility and lends nothing to the larger conversation. This trend of behaviour is not dissimilar to people who will loudly declare inaccurate knowledge and refuse to be corrected. It makes you seem ignorant, and often extremely pretentious.
Usually when I've formed a negative opinion about something I have no experience with, it's based on research or informed opinions from people I trust. If there's a book series I've chosen not to read, for example, I'll explain why based on the research I've done or feedback I received from friends who did read the series. This often leads to deeper discussions about content in literature and what either of us have found valuable in other books. It's valuable discourse.
So, when asked for your opinion on something you have no point of reference for, just say that. Don't dismiss something or embrace something in order to have an opinion to offer. It doesn't in any way benefit you, others' opinions of you, or the larger discussion being had.